1. Field of the Invention
The present invention relates to a tracking system and an autonomous mobile unit used therefor.
2. Description of the Related Art
A tracking system for controlling movement of autonomously moving unit (hereinafter also referred to as an “autonomous mobile unit”), such as a transfer robot, to track a moving object (hereinafter also referred to as a “master”), such as a human, has been used, for example, in factories. A well-known example of such a tracking system is a tracking apparatus that estimates the position of a master using ultrasonic waves.
This type of apparatus generally employs an echo scheme or a transponder scheme. In the echo scheme, only the autonomous mobile unit side has an ultrasonic transceiver, and a relative position of the master is measured by transmitting an ultrasonic wave to the master from the autonomous mobile unit side and receiving the ultrasonic wave reflected by the master at the autonomous mobile unit side. On the other hand, in the transponder scheme, each of the master and the autonomous mobile unit has an ultrasonic transceiver so that a relative position of the master is measured by mutually transmitting and receiving ultrasonic waves (for example, JP 7-31244B). In the transponder scheme, the master-side ultrasonic transceiver is called a transponder.
Conventional tracking devices, however, have experienced difficulty in estimating the position of the master if an obstacle blocking the ultrasonic wave exists on a direct path connecting the master and the autonomous mobile unit.
A method using a radar tracking apparatus is disclosed as a method for estimating the position of a object to be measured (target object) when an obstacle exists between the measured object and a device for measuring the position of the target object (JP 8-248125A). This method will be discussed with reference to FIG. 16.
In FIG. 16, a radar apparatus 202 is placed on a land that is inward of a coastline 201 so that the radar apparatus 202 looks out ships on the sea. An obstacle 203, such as an island, exists in a surveillance area of the radar apparatus 202, and the obstacle 203 forms a blind spot region 204. A primary reflector 205 for radar radio waves is set adjacent to the blind spot region 204. For the primary reflector 205, a large ship on the berth is utilized, for example. A ship under way, which is a tracked object, moves from a position 206 in a direction 207.
When the tracked object comes to a position 208 in the blind spot region 204, a radar radio wave reaches the primary reflector 205 through a propagation path 210, then reflects off the primary reflector 205, and reaches the tracked object at the position 208 through a propagation path 211. That radar radio wave reflects off the tracked object, travels via the propagation path 211, the primary reflector 205, and the propagation path 210, and reaches the radar apparatus 202, where it is received. In this case, a virtual image of the tracked object at the position 208 is displayed in the radar image at a position 209, which is behind the primary reflector 205. The position of the tracked object, which exists in the blind spot region, can be estimated from these pieces of information, utilizing the reflection path.
The method of JP 8-248125A, however, is a method used in a situation in which measurement parameters are limited, for example, in such a situation that the radar apparatus and the blind spot region of the measurement are almost fixed. For this reason, it is difficult to apply this method to a tracking system in which environment conditions change considerably from time to time.